In the case of high-frequency amplifiers particularly for transmitting a wideband signal such as a CDMA signal, there are special requirements that the gain characteristic and phase-response characteristic have to meet but that cannot easily be met by the active components. One possible way of linearizing these characteristics is to apply to the input signal of the amplifier pre-distortion that is the inverse of the distortion the signal will be subject to in the amplifier. To enable pre-distortion of this kind to be performed, it is necessary to determine the characteristics of the amplifier and the inverse characteristics needed for the predistortion. However, the method according to the invention is suitable not only for determining characteristics of a high-frequency amplifier, that can be looked upon as a two-port network, but also in general for determining parameters of an n-port network, such as a mixer for example that can be looked upon as a three-port network.
Published German patent application no. DE 198 13 703 A1 discloses a method of determining the characteristics of a non-linear power amplifier and for automatically tuning a pre-distorter inserted upstream of the power amplifier. In the method that is known from this printed publication, the input signal to the amplifier is split into an envelope curve and a reference carrier by means of a synchronous demodulator. The reference carrier is fed to the synchronous demodulator provided at the output of the power amplifier to allow phase-coherent demodulation into the in-phase component and quadrature component of the output signal to be performed with the reference carrier. A suitable indicator or display means is provided in this case to allow the amplitude characteristic and phase characteristic to be shown. At the same time, the characteristic curve that is obtained in this way is fed to a digital signal processor that so regulates the predistortion that as linear as possible an amplitude characteristic and as constant as possible a phase characteristic are produced. However, it is a prerequisite of this method, that is carried out while the amplifier is operating, that the input signal to the amplifier and the output signal are correlated with one another. If the amplifier is measured not in an operational situation but on a test rig with for example a one-channel measuring device and without the complicated and costly synchronous demodulation described in DE 198 13 703 A1, then there is a measurement problem in that, due to different trigger times for example, there is no precisely defined temporal correlation between the amplifier's input and output signals.
A need exists therefore to specify a method of determining parameters of an n-port network in which the measurement of an output signal train can be performed without referencing to the timing of an input signal train and it is in particular possible for complicated and costly synchronous demodulators for generating a reference carrier to be dispensed with.